Improve text only rendering time
It seems like you have done some of things I would have already suggested.


Use Blender-internal or OpenGL render.
Reduce anti-aliasing samples.
Use a shadeless material
Don't use any lamps.
Tweak text resolution (for fewer triangles)
Experiment with tile size (larger tiles may give better performance)
Lock the user interface while rendering.
Render from the command line. (means you can't use OpenGL easilysee  Render with openGL from the command line? )
Check which formats save to disk fastest (PNG with low compression is quite good).